The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED can be categorized into two major types according to the driving methods, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor (TFT) matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
The AMOLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. Most of the present Integrated Circuits (IC) only transmit voltage signals. Therefore, the AMOLED pixel driving circuit needs to accomplish the task of converting the voltage signals into the current signals. The traditional AMOLED pixel driving circuit generally is 2T1C, which is a structure comprising two thin film transistors and one capacitor to convert the voltage into the current.
As shown in FIG. 1, which shows a 2T1C pixel driving circuit employed for AMOLED according to prior art, comprising a first thin film transistor T10, a second thin film transistor T20 and a capacitor Cs. The first thin film transistor T10 is a drive thin film transistor, and the second thin film transistor T20 is a switch thin film transistor, and the capacitor Cs is a storage capacitor. Specifically, a gate of the second thin film transistor T20 is electrically coupled to a scan signal voltage Vsel, and a source is electrically coupled to a data signal voltage Vdata, and a drain is electrically coupled to a gate of the first thin film transistor T10 and one end of the capacitor Cs; a source of the first thin film transistor T10 is electrically coupled to an alternating current power supply voltage Vdd, and a drain is electrically coupled to an anode of the organic light emitting diode D; a cathode of the organic light emitting diode D is electrically coupled to an earth; the one end of the capacitor Cs is electrically coupled to the drain of the second thin film transistor T20, and the other end is electrically coupled to the source of the first thin film transistor T10.
Please refer to FIG. 2. FIG. 2 is a sequence diagram corresponding to the circuit in FIG. 1. As shown in FIG. 2, the working procedure of the 2T1C pixel driving circuit shown in FIG. 1 is divided into four stages, which specifically are: 1. a reset stage, the scan signal voltage Vsel provides high voltage level for controlling the second thin film transistor T20 to be activated, and the data signal voltage Vdata provides a first reference voltage Vref1 to the gate of the first thin film transistor T10 via the second thin film transistor T20. The gate voltage of the first thin film transistor T10, Va=Vref1, and the first thin film transistor T10 is activated, and an alternating current power supply voltage Vdd provides low voltage level Vdl, and then, the source voltage of the first thin film transistor T10, Vb=Vdl; 2. a threshold voltage detection stage, the scan signal voltage Vsel provides high voltage level for controlling the second thin film transistor T20 to be activated, and the data signal voltage Vdata provides a second reference voltage Vref2 to the gate of the first thin film transistor T10 via the second thin film transistor T20, and Vref2<Vref1. The gate voltage of the first thin film transistor T10, Va=Vref2, and the first thin film transistor T10 is activated, and the alternating current power supply voltage Vdd provides high voltage level, and the source voltage Vb of the first thin film transistor is raised that Vb=Vref2−Vth, and Vth is a threshold voltage of the first thin film transistor T10; 3. a threshold voltage compensation stage, the scan signal voltage Vsel provides high voltage level for controlling the second thin film transistor T20 to be activated, and the data signal voltage Vdata provides a data signal voltage Vdata to the gate of the first thin film transistor T10 and the capacitor Cs via the second thin film transistor T20. The gate voltage Va of the first thin film transistor T10=Vdata, and the first thin film transistor T10 is activated, and an alternating current power supply voltage Vdd provides high voltage level, and, the source voltage Vb of the first thin film transistor T10 is changed to Vb=Vref2−Vth+ΔV, and ΔV is the influence generated by the data signal voltage Vdata to the source voltage of the first thin film transistor T10; 4. A drive stage, the scan signal voltage Vsel provides low voltage level, and the second thin film transistor T20 is deactivated. With the storage function of the capacitor Cs, the gate voltage of the second thin film transistor T20 can be maintained to be the data signal voltage, Va=Vdata so that the first thin film transistor T10 is in an activated state. The source voltage of the first thin film transistor T10, Vb=Vref2−Vth+ΔV, and the gate-source voltage of the first thin film transistor T10, Vgs=Va−Vb=Vdata−Vref2+Vth−ΔV. The threshold voltage of the drive thin film transistor can be compensated. However, drawbacks of complicated data signal voltage and short compensation time exist in the 2T1C pixel driving circuit shown in FIG. 1.